To cite package 'frontier' in publications use:

Tim Coelli and Arne Henningsen (2020). frontier: Stochastic Frontier Analysis. R package version 1.1-9. https://CRAN.R-Project.org/package=frontier.

Corresponding BibTeX entry:

  @Manual{,
    title = {frontier: Stochastic Frontier Analysis},
    author = {Tim Coelli and Arne Henningsen},
    year = {2020},
    note = {R package version 1.1-9},
    url = {https://CRAN.R-Project.org/package=frontier},
  }